Separation band detecting circuit in an automatic program selection record player

Abstract

A separation band detecting circuit is used in an automatic program selection record player which has a photodetector which moves together with a tone arm and comprises a light-emitting element for emitting light against the surface of the record and a photosensitive element for receiving the light reflected at the record surface and producing an output in accordance with the light quantity of the received reflected light. The detecting circuit comprises an operational amplifier supplied with the output of the photodetector to its one input terminal, and a time constant circuit connected to the other input terminal of said operational amplifier, for giving a threshold level with respect to the operational amplifier and giving a time constant with respect to the level fluctuation in the threshold value. The time constant circuit varies the threshold level with a time constant, according to an input signal level to the one input terminal of the operational amplifier. The operational amplifier inverts its output when the relationship between amplitudes of the input signal level and the threshold level reverses.

What is claimed is: 
     
       1. A circuit for detecting separation bands between bands recorded on a record, said detecting circuit being in an automatic program selection record player, said record player having a photodetector and a tone arm which move together, said photodetector having a light-emitting element for directing light against the surface of the record and a photosensitive element for receiving the light which is reflected from the record surface and for producing an output signal which is in accordance with quantity of the reflected light which is received by said photosensitive element, said detecting circuit comprising: an operational amplifier having first and second input termnals, said first input terminal being supplied with the output signal of said photodetector, said output signal including a D.C. component; and   a time constant circuit having a predetermined time constant and including a diode which is coupled between said first and second input terminals of the operational amplifier, said second input terminal being coupled through said time constant circuit to a potential source;   means including said time constant circuit for supplying a threshold voltage to said second input terminal of the operational amplifier, said threshold voltage undergoing a gradual voltage level change with said predetermined time constant to follow a voltage level change in the output signal of said photodetector, and   means including said operational amplifier for inverting an output signal in response to a sudden large level change in the output signal of said photodetector, said sudden large level change occurring with a period which is shorter than said predetermined time constant and accompanying a voltage change to a voltage level which exceeds the level of said threshold voltage.   
     
     
       2. The detecting circuit as claimed in claim 1 in which said first and second input terminals respectively are inverting and non-inverting input terminals of said operational amplifier, and said time constant circuit comprises: a charging and discharging capacitor coupled to the non-inverting input terminal of said operational amplifier; a first resistor coupled in parallel with said capacitor, for determining a charging time constant together with said capacitor; and a second resistor coupled to the inverting input terminal of said operational amplifier, for determining a discharging time constant together with said capacitor, said predetermined time constant being the charging time constant of said time constant circuit. 
     
     
       3. The detecting circuit as claimed in claim 2 in which said diode is connected in the forward direction from the non-inverting input terminal to the inverting input terminal of said operational amplifier. 
     
     
       4. The detecting circuit as claimed in claim 2 in which said diode has a predetermined biasing voltage, and said threshold voltage follows the voltage level change in the output signal of said photodetector with a voltage difference which is equal to said predetermined biasing voltage. 
     
     
       5. The detecting circuit as claimed in claim 1 in which said predetermined time constant of the time constant is variably set according to the operational mode of the record player so that the output signal of said photodetector exceeds said threshold voltage when said photosensitive element receives the light which is reflected from a separation band on the record surface. 
     
     
       6. The detecting circuit as claimed in claim 2 in which said charging time constant is a value in a range of approximately 5 milliseconds to 50 milliseconds, and said discharging time constant is a value in a range of substantially 0.01 milliseconds to 0.5 milliseconds.